This invention relates to magnetic resonance imaging.
A typical magnetic resonance imaging apparatus is shown in FIG. 1. A patient 1 on a couch 2 is slid into the bore 3 of an annular electromagnet, typically a superconducting electromagnet. A main magnetic field is generated in alignment with the axis of the bore, and gradient coils (not shown) are provided to set up magnetic field gradients for example along the z-direction along the axis of the bore, and along x and y directions in the radial plane. A transmit coil 4 surrounds the patient and transmits pulses of r.f. energy to excite to resonance magnetic resonance active nuclei such as protons in the region of the patient to be examined. This transmit coil 4 is normally surrounded by an r.f. shield coil 5 to shield the bore 3 of the electromagnet from extraneous unwanted r.f. signals. The transmit coil 4 can be also be used to receive the magnetic resonance signals which result from the resonating protons in the region of interest, although a separate receive coil is often provided. For many examinations, a coil placed in the vicinity of the surface of the patient is used to receive the magnetic resonance signals, such as the coil 6 (shown on an enlarged scale in FIG. 2).
To increase the signal-to-noise ratio, it has been proposed to use an array of coils in the vicinity of the surface of the patient, for receive purposes. There are various reasons for this, all of which usually stem from a desire to reduce the scanning time required to build up an image of the region of interest of the patient.
Thus, it could be that the sensitive region of each coil of the array matches better the region it is desired to image than would one large receive coil, and more signal can therefore be collected from the region of interest. It could also be that the localised view of the region it is desired to image obtained from each coil of the array can be used to advantage to reduce the number of encoding steps (WO-A-98/21600 and European Patent Application No. 1 014 102).
In many cases, it would be desirable to separate adjacent coils of the array by means of screens. The field of view of each coil is then more precisely defined.
The applicants are aware that microstructures with magnetic properties comprising an array of capacitive elements made from non-magnetic conducting material can exhibit magnetic permeability at radio frequencies (IEEE Transactions on Microwave Theory and Techniques, Vol. 47, No. 11, November 1999, Magnetism from Conductors and Enhanced Non-Linear Phenomena by J B Pendry, A J Holden, D J Robbins and W J Stewart, and International Patent Application WO-A-00/41270).
The invention provides magnetic resonance apparatus, in which resonance is excited in use in magnetic resonant active nuclei in a region of an object in the presence of a main magnetic field, which magnetic resonance apparatus includes a structure with magnetic properties forming a screen to screen r.f. flux, the structure comprising an array of capacitive elements, the array exhibiting a predetermined magnetic permeability in response to incident electromagnetic radiation lying within a predetermined frequency band, wherein each capacitive element includes a conducting path and is such that a magnetic component of the electromagnetic radiation lying within the predetermined frequency band induces an electrical current to flow around said path and through said associated element, wherein the spacing of the elements is less than the wavelength of the radiation within the predetermined frequency band, and wherein the size of the elements and their spacing apart arm selected such as to provide a magnetic permeability appropriate to that of a screen to electromagnetic radiation in that predetermined frequency band.
This enables the field of view of each coil, and the properties of the array as a whole, to be varied
Such material is described in the IEEE article and in the International Patent Application WO-A-00/41270 referred to, the latter being incorporated herein by reference.
Preferably, at least one dimension of each capacitive element is less than the wavelength corresponding to the frequency of electromagnetic radiation exciting magnetic resonance.
The spacing of the elements may be less than one half, preferably less than one fifth of the wavelength of the radiation at the magnetic resonance frequency. Further advantages may flow from making the element spacing less than one tenth, or less than one hundredth of the wavelength of the radiation at the magnetic resonance frequency.
The screen may be capable of exhibiting a magnetic permeability having a zero or negative real part to electromagnetic radiation within the predetermined frequency band. The screen may be capable of being switched between this value, and the permeability of free space.
The screens of the structure with magnetic properties are however capable of use for any purpose in magnetic resonance apparatus, which may be magnetic resonance imaging apparatus or magnetic resonance spectroscopy apparatus, since the screens are non-magnetic in a steady magnetic field.
The screen will typically be tuned to the magnetic resonance frequency, but could be tuned to one or more different frequencies, for example, if it was desired to prevent interference of a particular external r.f source with a magnetic resonance experiment.
The invention also provides magnetic resonance apparatus, in which resonance is excited in use in magnetic resonant active nuclei in a region of an object in the presence of a main magnetic field, which magnetic resonance apparatus includes the said microstructured magnetic material to shield the region from extraneous r.f. energy.
The invention also provides magnetic resonance imaging apparatus, in which resonance is excited in use in magnetic resonant active nuclei in a region of an object in the presence of a main magnetic field, which magnetic resonance imaging apparatus includes the said microstructured magnetic material to shield regions of the object it is not desired to image from r.f. excitation applied to the region it is desired to image.